First and foremost: Thanks to the hivemind for helping with this! And Thanks to @Linnet for first compiling the list and creating the post!
Remember the first time someone screamed "BRDM! It's a BRDM!", at you and you had no damn clue how you were supposed to react to this? Or what to look for? The list of abbreviations is damn long (and this one by no means complete! If you see something missing, suggest it in the comments please! Missing an abbreviation but you're not sure what it means? Add it, too! Chances are somebody will fill it in) and I'm 100% convinced that nobody knows every single one of them. This list - click here for the entire one in alphabetical order - may prove itself helpful for some of the FNGs in the future. Running away tactically retreating from something makes more sense when you know what the hell you're running from.
This list will be separated into 3 subsections, namely: Weapons, Vehicles and General Terminology.
NOTE: This part only focuses on weapons. You can find Vehicles here and General Terminology here
Weapons:
A:
AA – Anti-Air
AAA - Anti-Air-Artillery (AAA or Triple A)
AK – Avtomat Kalashnikova
AP – Anti-Personnel OR Armour-Piercing
APFSDS – Armour Piercing Fin Stabilised Discarding SABOT (utilized by big guns, usually tanks)
AR – Assault Rifle OR Automatic Rifle
AT – Anti-Tank
ATG – Anti-Tank Gun
ATGM - Anti-Tank-Guided-Missile
C
Cal - Caliber
D
DMR – Designated Marksman Rifle
DMS - Designated Marksman Scope
DShKM - Heavy (infantry) machine gun, firing 12.7×108mm
E:
EPR - Enhanced Performance Round
F:
Frag – Fragmentation grenades
G:
GL - Grenade Launcher
H:
HAT - Heavy Anti-Tank; Reloadable, guided launchers (usually refers to the launcher a Squad brings)
HBAR – Heavy Barrel Automatic Rifle
HE - High-Explosive
HEAT – High-Explosive Anti-Tank
HEDP - High Explosive Dual Purpose
HP - High Powered OR High Penetration
HMG – Heavy Machine Gun (think M2 or DShK)
I:
IR – Infra-Red
IR-DIM - Infra Red Dim, a type of tracer round that is only visible to people wearing NVGs, and will not start to illuminate until after having flown ~100+ yards. (To not give gunner position away)
L:
LMG – Light Machine Gun
LAT - Light-Anti-Tank (usually refers to the launcher a Squad brings)
LHAT - Laser-Homing-Anti-Tank
M:
Mag – Magazine
MAT - Medium-Anti-Tank (usually refers to the launcher a Squad brings)
MMG – Medium Machine Gun - An autorifle with 7.62 caliber
MPAT - Multi-Purpose Anti Tank
R:
RCO – Rifle Combat Optics
RPG – Rocket Propelled Grenade
S:
SAW - Squad-Automatic-Weapon (refers to the automatic rifle or machinegun utilized by a Squad)
SOF - Special Operations Forces
SOST - A special type of tungsten headed round. It’ll ‘detonate’ after a moment, scattering shrapnel within a persons body
T:
TOW - Tube-launched-Optically-tracked-Wire-guided (the missile goes where you look)
U:
UGL - Underslung-Grenade-Launcher (weapon-mounted)
updated last on June 7th 2021 by @Silberjojo(send any feedback to him)